The present application relates to a method for disposal of dilute aqueous solutions of metal cyanide complex compounds. More particularly, the present application discloses a method for disposal of metal cyanide complex containing foul waters produced from a fluidized catalytic cracking process.
Many naturally occurring hydrocarbon oils, such as petroleum, shale oil, tar sand oils, and their fractions contain nitrogen compounds which under proper conditions react with metals, particularly iron, and carbon to form metal cyanide complex compounds. Reaction conditions which favor formation of such metal cyanide complex compounds include contact of nitrogen containing oil with metals such as iron at elevated temperatures of about 700.degree. F and higher in a reducing atmosphere. The metal cyanide complex compounds so produced are at least slightly soluble in water, and commonly are recovered from refinery processes as components of various foul water streams, often in admixture with other undesirable reaction products such as phenol and hydrogen sulfide. Protection of environmental quality requires that such metal cyanide complex compounds be prevented from entering natural waters.
The major refining unit process wherein such metal cyanide complex compounds are formed is the Fluidized Catalytic Cracking Process. In this process, a small percentage of nitrogen compounds contained in the hydrocarbon charge is converted into metal cyanide complex compounds. These metal cyanide complex compounds are generally recovered in solution with foul waters from downstream process equipment employed in conjunction with fluidized catalytic cracking units. That is, metal cyanide complex compounds are commonly found in foul waters from fluidized catalytic cracking unit main fractionator overhead accumulation drums, compressor after-cooler receiver drums, debutanizer fractionator overhead accumulation drums, and other sources wherein liquid water is separated from hydrocarbon products of the fluidized catalytic cracking process.
Although some water may enter the fluidized catalytic cracking process with hydrocarbon charge, and some may be a reaction product, by far the largest volume of water enters the fluidized catalytic cracking process as stripping steam and fluidization steam employed to obtain performance of the fluidized catalytic cracking process. Such steam entering the reaction side of a fluidized catalytic cracking process is carried overhead with hydrocarbon product vapors and is condensed to liquid water in downstream processing equipment whenever conditions of temperature and pressure allow. Such condensed water may contain water soluble compounds formed in the fluidized catalytic cracking reaction, including hydrogen sulfide, phenols, and the metal cyanide complex compounds which are the subject of the present invention. Such condensed foul waters are accumulated and withdrawn from refinery process equipment for subsequent disposal.
The metal cyanide complex compounds are present in such foul waters in low concentration, generally less than 100 parts per million by weight (wppm), and usually about 2-10 wppm. These metal cyanide complex compounds are difficult to analyze, giving neither the reaction for the particular metals nor for cyanide in standard tests, and are considered to be one or more metal complexes of ferrous and/or ferric cyanide. As many metals, particularly the alkaline, alkaline earth, and iron group metals can form such metal cyanide complexes with cyanide compounds such as ammonium cyanide, ferrous cyanide, and ferric cyanide, and since many of these metals may be present in the reaction zone of a fluidized catalytic cracking unit, a great number of such metal complex cyanides are potentially present in the recovered foul waters.
Many methods have been proposed for disposal of cyanide compounds contained in industrial waste waters. These methods include biological reduction, chemical reduction, and incineration. Biological processes were found to be insufficiently controllable to insure reduction of the cyanides within a reasonable time. Chemical processes, such as reaction of the metal cyanide complexes with ferrous sulfate in an acidic or alkaline medium and subsequent removal of the cyanide as a Prussian Blue sludge, are effective. However, chemical costs are substantial and the sludge disposal becomes an environmental problem. Incineration of the cyanide containing foul water in the presence of a flame and oxygen is effective for disposal of the metal cyanide complex containing foul water. Additionally, phenols may be disposed of in an incinerator. The major draw-back to incineration of such foul water is a special incinerator must be constructed and fuel for the incineration must be supplied, all at substantial cost.
More than 1 year prior to this application, foul water containing hydrogen sulfide, phenols, and metal cyanide complex compounds was employed as spray water in a fluidized catalytic cracking unit regenerator for cooling the cyclones and dilute phase temperature control. This was undertaken as a water saving method, and disposal of cyanides was not contemplated, consequently, only a portion of cyanide containing foul water available was disposed of in this matter. Disadvantages of this method of cyanide disposal include contact of cracking catalyst with the cyanide compounds, which are known to have a deleterious effect upon the catalyst. Also, use of the unstripped foul waters places additional sulfur in the flue gas exiting the fluidized catalytic cracking unit.